Drawbar Vs. Belt Horsepower

Agricultural Tractor Industry Standards

• Although various standard development organizations now exist for setting specialized tractor standards, the initial SDO was the Nebraska Tractor Test Laboratory, which was launched in 1920 by the Nebraska Department of Agriculture. It has now become the official test station for the Organization of Economic Co-operation and Development, representing tractor standards throughout most of the world. NTTL test reports are used by manufacturers and dealers to promote their products, and by farmers for comparing different brands by the amount of work the tractor can perform, and how much fuel is required.

Engine Horsepower

• Engine horsepower is the least important factor taken into consideration by most dealers and farmers when comparing brands. Engine horsepower is usually measured at the crankshaft; therefore power losses caused by oil and coolant pumps, hydraulic systems, and gears are not listed. Even though these losses affect engine horsepower, the engine horsepower figures are always shown to be higher than belt and drawbar horsepower – which is the net power available for agricultural tasks.

Belt Horsepower

• Belt and power take-off horsepower is used to drive various farm implements and equipment requiring external power. The NTTL measures this power at the pulley driveshaft or the PTO shaft, and both outputs are usually within a fraction of one horsepower at their respective measuring points. This is the net power available at the PTO or belt pulley after subtracting power losses incurred by essential mechanical, hydraulic and electrical equipment attached to the engine, such as alternators and coolant pumps.

Drawbar Horsepower

• Drawbar horsepower is used as a direct comparison between tractors used for hauling operations and other tasks such as plowing, cultivating and disc harrowing. Since powertrain losses are taken into account, drawbar horsepower is sometimes referred to as “power delivery efficiency.” This represents the percentage of power produced by the tractor’s engine that is available as tractive power delivered through the drawbar. NTTL tests, however, are carried out on a level tarmac surface, therefore actual delivered drawbar horsepower can vary depending on terrain, wheel size, soil compaction and tractor weight.